Bump ancient OkHTTP v2.7.5 to current 4.x and Retrofit 1.9.0 to current 2.9.0

While I was working on our new fineract-client which through Retrofit uses a current version 4.x of https://square.github.io/okhttp, I've noticed that fineract-provider itself is still stuck on an ancient OkHTTP 2.7.5 version.

While client and server COULD be on different versions, it seems to make sense to me to align these versions.